In recent years, extensive studies have been made to put a lactic polymer having an excellent biodegradability into wide application as a general-purpose polymer. Many studies have been made of process for the preparation of such a lactic polymer. Further, many proposals for the process for the preparation of such a lactic polymer have been applied for patent. However, the conventional lactic acid or polylactic acid which is a polymer of lactides, or copolymer of lactide with other monomers leave something to be desired in formability or moldability and transparency. The polylactic acid is disadvantageous in that it decomposes too fast to handle as a general-purpose resin except in special applications. It has thus been keenly desired to improve these polylactic polymers.
International Disclosure No. WO 91/02015 discloses copolymers of an aromatic polyester such as polyethylene terephthalate and polybutylene terephthalate with polyglycolide or polylactic acid and processes for the preparation thereof.
The preparation processes disclosed in the above cited patent include a process which comprises the reaction of monomers, i.e., lactide, butylene glycol and dimethyl terephthalate, and a process which comprises the reaction of polymers, i.e., ester exchange reaction of two polymers, i.e., polyglycolide and polybutylene terephthalate, at a temperature as high as 220.degree. C. However, preparation processes described in the examples are limited to the ester exchange reaction of polymers.
JP-A-4-504731 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process which comprises the polymerization of a lactide with a polyethylene terephthalate to prepare a blend polymer of a polylactic acid with a polyethylene terephthalate. Further, a technique for reacting a crystalline aromatic polyester with a lactone is disclosed in JP-B-48-4115 (The term "JP-B" as used herein means an "examined Japanese patent publication") and JP-B-48-4116. In accordance with these methods, a crystalline aromatic polyester is reacted with a lactone, particularly .epsilon.-caprolactone or .gamma.-valerolactone.
However, the process disclosed in JP-A-4-504731 is disadvantageous in that the softening point of the polyethylene terephthalate is as high as not lower than 220.degree. C., which is higher than the decomposition temperature of the lactide (185.degree. C.), giving a remarkably colored copolymer having an insufficient molecular weight. Further, the process which comprises the reaction of lactones disclosed in JP-B-48-4115 and JP-B-48-4116 is disadvantageous in that the resulting copolymer is opaque and flexible and thus is not preferred as a molding resin.
It is generally well known that the preparation process which comprises the reaction of monomers, i.e., the reaction of a dicarboxylic acid component or its esterified product with a diol component and a cyclic ester such as lactide, cannot provide a higher molecular weight. Further, the preparation process which comprises the reaction of polymers is impractical in that the decomposition temperature of the polylactic acid is much lower than the temperature at which the aromatic polyester such as polyethylene terephthalate and polybutylene terephthalate becomes fluid.
Moreover, the resulting lactic copolymer polyester is brittle and exhibits a poor transparency because the aromatic polyester is crystalline and exhibits a high melt temperature and a poor compatibility with other compounds. JP-A-63-145661 proposes a process for the preparation of a copolymer of a lactide with an aliphatic polyester which comprises the polymerization of .epsilon.-caprolactone to obtain a homopolymer which is then block-copolymerized with a lactide.
However, the above-proposed process which comprises the block copolymerization of a poly(.epsilon.-caprolactone) with a lactide is disadvantageous in that the resulting copolymer becomes cloudy and opaque. This is probably because that the poly(.epsilon.-caprolactone) block and the polylactic acid block in the copolymer are hardly compatible with each other and the aliphatic polyester in the poly(.epsilon.-caprolactone) chain reflects a high crystallinity to opacify the copolymer. Further, the copolymer thus obtained normally stays flexible despite of its relatively high glass transition point determined by differential thermal analysis.
To summarize these conventional techniques, polymers provided with a sufficient strength, heat resistance and thermal stability exhibit an insufficient flexibility and transparency. On the contrary, polymers provided with a sufficient flexibility and transparency exhibit an insufficient strength, heat resistance and thermal stability. Thus, polymers provided with properties satisfactory enough for resins to be formed into film or sheet have not yet been obtained.
Further, if a lactide to be incorporated as a residual monomer is used as a plasticizer to plasticize the polymer, the remaining lactide sublimates, scatters and then attaches itself to the apparatus during preparation, contaminating the apparatus. Moreover, the lactide disappears from the polymer during storage or while in use, the desired plasticizing effect disappears and things wrapped by the wrapping material can be contaminated.
If an ordinary plasticizer is used instead of lactide, it must be used in a large amount to attain a sufficient plasticizing effect. Thus, the plasticizer unavoidably bleeds out, and the problems such as the disappearance of the desired plasticizing effect during storage and the contamination of things wrapped by the wrapping material could not be solved. Accordingly, polymers provided with properties satisfactory enough for the application as wrapping materials have not yet been obtained.